This invention concerns a traction unit for a drawing machine.
To be more exact, the invention concerns a traction unit of a drawing machine, the traction unit being positioned downstream of the drawing die and having the purpose of drawing the metallic stock in a continuous, regular manner with a required drawing action so that the metallic stock is drawn.
The invention is applied advantageously to the field of the drawing of metals.
Devices to draw metals, as is known, comprise substantially an element or drawing die to reduce the section of the stock and means to draw lengthwise the metallic stock to be subjected to the drawing process. These drawing means can apply their action to the stock in a discontinuous or a continuous manner.
Discontinuous drawing means generally include gripper means with one or more grippers, which clamp the stock and draw it, but these means entail the problem that the section of the stock is deformed at the gripping points and the stock is marked when the gripper is changed. In fact, these gripper drawing means develop speeds which are no longer adequate.
Continuous drawing means generally comprise a device consisting of two opposed chains, which operate respectively on the two opposite sides of the stock at the segment which has already passed through the die.
These chains normally consist of a series of links or tracks, connected together in various ways or else not connected together but merely positioned alongside each other. The chains are set in continuous rotation and, in cooperation with rigid guides, clamp the stock to be drawn and draw it in a straight line.
The perpendicular pressure exerted by the links on the drawn stock transmits the lengthwise motion of the chains to the stock and applies the necessary drawing action for the drawing of the stock.
The cooperation between the rigid guides and chains together with the lengthwise movement caused by the rotation of the chains produces by contact a perpendicular pressure of the links of the chains against the stock being drawn.
It should be borne in mind that this perpendicular pressure reaches and exceeds values about ten times greater than the value of the drawing action required for the drawing process in a lengthwise direction.
The reciprocal contact between the rigid guides and the chains leads to problems of wear due to the great sliding friction generated between the two flat surfaces, since a given contact takes place at least at the segment where the chains slide opposite to each other.
Various solutions to restrict these problems have been disclosed and, for the most part, provide for the use of rotary elements such as balls or rollers placed between the rigid guides and the chains so as to eliminate the sliding friction and replace it with a revolving friction of a much lower value.
U.S. Pat. No. 2,642,280 discloses a solution which, to reduce the friction, replaces the rigid guides with a series of rollers fitted in succession in the manner of a beam on ball bearings or bearings held on stationary supports.
These rollers are constantly stressed by considerable loads and have a limited life owing to the shear stresses and flexural stresses which they undergo owing to the loads acting on them.
Moreover, as the rollers are fitted so as to form a beam, the loads cause a central bending of the rollers and possible deformation of the lateral supports together with an incorrect contact between the chain and the rollers.
Furthermore, these rollers, precisely because they are positioned in a stationary position, undergo differentiated wear, with the outcome that the perpendicular pressure exerted on the drawn stock is not constant in the long term in a lengthwise direction.
U.S. Pat. No. 2,797,798 discloses an embodiment which entails substantially the same problems as those detailed above, even though the stationary lateral supports of the rollers are brought closer and therefore the bending moment generating the central bending is partly reduced.
In this embodiment, however, it is likely that the system will seize up during working owing to the smaller lateral play between the rollers and their respective stationary lateral supports.
U.S. Pat. No. 3,945,547 discloses an embodiment in which two rings of balls cooperate with the sides of the links of each single chain. These rings of balls circulate continuously, thus obviating differentiated wear of the balls, but in this way the lateral ends of the links are stressed by punctiform loads which, owing to the high pressures involved, deform and incise those lateral ends, thus no longer providing the required characteristics of the drawing action.
Moreover, as the two rings of balls are free to move independently of each other, the coordinated positioning of the balls on one side and the other side of the individual link is not ensured. This fact leads to imbalances of the pressure exerted on the link itself and therefore on the drawn stock.
According to a variant of this embodiment the rings of balls are replaced by rings of rollers connected to each other, In this case too the rollers cooperate with the lateral ends of the links, thus overcoming the problem of punctiform loads, but with this variant there still remains the problem of coordinating the movements of the two rings so as to ensure that the rollers acting on one side and the other of the links are always coaxial.
Besides, this variant entails the problem of increasing the overall bulk of the device owing to the overall lateral dimensions of the rollers. Moreover, in this case, seeing that the rollers take up more space than the balls, either the width of the central cantilever part of the link has to be reduced or the lateral supports of the link have to be enlarged.
With the first of these alternatives the maximum load which can be borne by the link is reduced or the specific load is increased, with the resulting occurrence of deflections, which cause the supports of the links to work only on the edges of the rollers and not on their whole surface, as required.
With the second alternative the overall bulk of the chain and of the whole traction unit is increased. In this case too it is necessary that the two rings of rollers should be always aligned to prevent misalignments.
JP-A-58-154412 discloses a traction unit for a drawing machine with three elements, namely a traction belt with an indented back that draws a metallic chain which presses on pressure rollers; each pressure roller is independent and is guided only in grooves positioned at the sides.
This disclosure cannot be applied to modern drawing machines, which require very short downtimes for maintenance work and high working speeds. In fact, this teaching does not permit loads misaligned in relation to the centre line nor uneven wear of the contact chain.
Moreover, this embodiment requires that there should be no constructional faults such as unevenness of the rollers, unlevel guides or an uneven thickness of the chains. In fact, if any of these shortcomings occur, the rollers become positioned crosswise and block the system.
This situation is possible owing to the necessary play incorporated in the grooves of the guides.
Moreover, as the motive load acts on the rubber teeth of the chain, these teeth are stressed with unbearable values.